NOISE AND VIBRATION CONTROL Episode 7 potx

normally be obtained from a manufacturer, but if the equipment is not selected the fan power levels can be determined from the method given in appendix C. The diffuser or grille sound power levels can be determined approximately from a manufacturer’s catalog for an identical or similar type of outlet. Similarly, with a variable volume system the sound power level for the air discharge of an air terminal unit, such as a VAV, or FPT, can be obtained from a manufacturer’s catalog for a given operating condition. For step (3) the attenuation provided by unlined and lined ducts, by sound attenuators, elbows, branches, and end reflection are added together to find the total insertion loss (IL) applicable to the control of the fan sound power as it propagates along the duct path between the fan and the air outlet. Similarly, for the prediction of the noise level in the space served caused by duct transmitted air discharge noise from an air terminal unit it is necessary to determine the insertion loss of the duct distribu- tion system between the terminal unit and the room air outlet. This IL will consist of the attenua- tion of any unlined or lined ductwork, elbows, duct branches, or splits, and end reflection although this latter will not be significant for air terminal unit noise. In step (4) the sound power level of all sources contributing to the sound power at the air outlet are determined and combined to find the total sound power level, in octave bands, at the air outlet. For step (5) it is necessary to determine the Room Factors and the “Rel SPLs” (see Chapter 3) for the space served and apply the “Rel SPL” to the sound power levels at the air outlet for each source to obtain the sound pressure level produced by that outlet at any location in the space served. For step (6) the resulting sound pressure levels are compared to the selected criteria to determine if additional sound attenuation is necessary. 7-6. Calculation Example. In this example the noise control requirements for an air distribution system serving a classroom as shown in figure 7-2 are calculated. A fan with TM S-805-4/AFJMAN 32-1090 forward curved blades delivers 20,000 cubic feet per minute (cfm) to a number of classrooms and offices against a static pressure of 2.5 in. of water with 12 brake horse power. The main supply duct has dimensions of 60 x 24 inches resulting in an air velocity of 2000 ft/min. The closest class room, which has dimensions of 24 x 24 x 10 ft, is supplied by a duct branching off from the main header duct. The classroom air is delivered from four diffusers, 10 inch in diameter each, mounted in the ceiling, each delivering 500 cfm. Thus the total air supplied by the branch duct is 2000 cfm, with an air velocity of 1000 ft/min. in the 12 x 24 in. duct. The only acoustical material applied to the room surfaces is a suspended acoustical ceiling representing approximately 25% of the room sur- faces. In this example it is assumed that the entire duct system is internally lined with one inch thick sound absorptive insulation, and the duct cross- sectional flow area is given by the dimensions stated in the schematic figure. The tabulated results for this example are as follows: a. Step (1). In this step an NC 30 is selected as the sound pressure level design criteria. b. Step (2). In this step the sound power level (Lw), in dB re 10-12 watts, of the supply fan and the diffusers are determined. (1) Fan Lw. From equation 10-5 and table 10-13. Octave Band Center Frequencies 63 125 250 500 1k 2k 4k Fan Kw 47 43 39 36 34 32 28 10log(cfm) 43 43 43 43 43 43 43 BFI. 2 201og(p) 8888888 Eff. Corr. 6666666 Total Lw of Fan 104 100 96 95 91 89 85 (2) Diffuser Lw, w/o damper. From suppliers catalog. Table 7-6. Losses Caused by Duct Elbows. Duct Diameter Octave Band Frequency (Hz) (inches) 63 125 250 500 1000 2000 4000 8000 Lined elbows 5 to 10 0 0 1 2 3 4 6 8 11 to 20 0 1 2 3 4 6 8 10 21 to 40 1 2 3 4 5 6 8 10 41 to 80 2 3 4 5 6 8 10 12 Unlined elbows All sizes 1 2 3 3 3 3 3 3 7-9 TM 5-805-4/AFJMAN 32-1090 Table 7-7. Representative IL Values for Sound Attenuators. Type Length Octave Band Center Frequency (Hz) in Ft. 63 125 250 500 1000 2000 4000 8000 Low 3 2 4 8 14 21 22 14 11 Pressure 5 3 5 11 23 31 34 19 13 Drop 7 5 9 16 32 41 43 24 16 10 6 15 22 37 52 53 35 23 Standard 3 3 6 13 23 33 33 23 14 Pressure 5 4 11 20 37 44 44 36 22 Drop 7 6 13 28 42 47 47 45 30 10 8 15 35 50 56 57 57 40 Octave Band Center Frequencies 63 125 250 500 1k 2k 4k Lw of One Diffuser N/A 43 37 35 40 40 23 c. Step (3). In this step the total attenuation for fan noise provided by the duct system including the lined ductwork, the duct branches, the elbows, and the end reflection loss are determined. This consists of determining the insertion loss (IL) for each element and then summing all of the inser- tion losses. (1) Lined Ductwork IL in dB. The insertion losses from each rectangular duct element is deter- mined from tables 7-2 and 13-3. The results of each element is summed by octave bands to pro- vide the total duct attenuation in dB in each octave band. Octave Band Center Frequencies 63 125 250 500 lk 2k 4k 20' of 24x60 3 4 7 18 32 30 30 9’ of 12x24 3 2 5 12 31 27 19 10’ of 12x16 4 3 7 16 41 41 28 3' of 10” 0 1 2 5 6 6 5 Total 10 10 21 >50 >50 >50 >50 It should be noted that table 7-3 does not contain entries for the 24x60 and 12x16 ducts. The attenu- ation values for these ducts are obtained by inter- polation. For example the attenuation for the 24x60 duct is the average value of the 24x48 and 24x72 ducts. For the 24x60 duct the full 20 feet of length is used since the elbow breaks the length into two lengths less than 10 feet each. Also it should be noted that the total attenuation is the 7-10 sum of the attenuation due to the internal lining (table 7-3) and the natural attenuation (table 7-2). The attenuation for the 10” round duct was ob- tained from a suppliers catalog. The total attenua- tion for all of the duct elements is limited to approximately 50 dB because this is usually the maximum that can be obtained in a connected system due to structural flanking down the duct wall. (2) Branches (To one diffuser). The branch attenuation is determined by equation 7-2 or table 7-4. With a branch area of 2 sq. ft. (i.e. 24x24) and the area after the branch of 10 sq. ft. (i.e. 24x60) the area ratio of the branch is 2/(10+2) or 0.167. The sound power loss at for the take-off in the corridor is approximately 8 dB in accordance with equation 7-2. The power division in the “T” and diffuser take-off are determined in a similar fash- ion and are approximately 3 dB each (i.e. 50% each way). Therefore the total attenuation due to all the branching is approximately 14 dB in all of the octave bands. Octave Band Center frequencies 63 125 250 500 1k 2k 4k Branch att. in corridor 888888 “T” in room 3333333 Diffuser Take-off 3 3 3 3 3 3 3 Total (dB) 14 14 14 14 14 14 14 (3) Four elbows. There are four elbows be- tween the fan and the classroom. The attenuation of each of these can be found from table 7-6. The first elbow is the 24x60 inch elbow that goes from the vertical to the horizontal at the fan outlet. For this elbow the duct diameter used is 24 inches since this is the dimension in the plane of the turn. The second elbow is a 60x24, for this elbow the dimension is 60 inches. The third elbow is the “T” from 12x24 to 12x16 over the classroom. In (2) above a power division was taken for this “T” fitting, however since some energy is also reflected from the “T” it also acts like an elbow. For the “T” the characteristic dimension is 24”. And the final elbow is the 12x16 over the class room. For this elbow the characteristic dimension is 16”. The attenuations for each elbow and the total attenua- tions for all of the elbows is given below. Octave Band Center Frequencies 63 125 250 500 1k 2k 4k 24x60 1234568 60x24 23456 8 10 24x12 1234568 16x12 0123468 Total IL 4 8 12 16 20 26 34 (4) End loss. The end reflection loss is taken from table 7-5 part A, where the diameter is 10 inches. Part A was used since the diffuser was mounted in an acoustical tile ceiling. If the ceiling was hard (gyp. bd., plaster, concrete, etc.) then part B would have been used. Octave Band Center Frequencies End Reflection (5) Total IL. ducted air supply 63 125 250 500 1k 2k 4k 16 10 6 2 1 0 0 The total insertion loss of the system is the arithmetical sum, in each octave band, of the insertion losses of (1) through (4) above. Octave Band Center Frequencies 63 125 250 500 1k 2k 4k Total line ducts 10 10 21 >50>50>50>50 Total branches 14 14 14 14 14 14 14 Total elbows 4 8 12 16 20 26 34 End Reflection 16 10 6 2 1 0 0 Total IL (all duct >50 elements) 44 42 53 >50 >50>50 Note again, the insertion loss is limited to approx- imately 50 dB. This is because flanking sound traveling within the duct walls can become a significant source of sound when the sound levels within the air stream have been attenuated a great deal. If attenuations greater than 50 dB are required, additional vibration breaks within the TM 5-805-4/AFJMAN 32-1090 duct would have to be evaluated. d. Step (4). In this step the total sound power at each of the two diffusers closest to the fan is determined. First the sound power transmitted to the room from the fan via the supply duct, is determined by subtracting the total attenuations (c. (5) above) from the total sound power of the fan (b. (1) above) by octave bands. These steps are shown below. Octave Band Center Frequencies 63 125 250 500 1k 2k 4k Total Fan Lw 104 100 96 95 91 89 85 (b.(1) above) Total Duct IL 44 42 53 >50>50>50>50 (c.(5) above) Resulting Fan Lw in Class- room 60 58 43 Then the sound power of the diffuser is added, logarithmically, to the sound power transmitted by the fan, as shown below. Octave Band Center Frequencies 63 125 250 500 1k 2k 4k Fan Lw in Classroom 60 58 43 Diffuser Lw 43 37 35 40 40 23 Total 60 58 44 35 40 40 23 This analysis provides the total sound power into the room from the operation of one diffuser. It also indicates the frequency range of the significant sources of sound. For example the 63, 125 and 250 Hz octave bands are dominated by the sound of the fan, whereas the level of the other octave bands are determined by the operation of the ceiling diffuser. This distinction is important since sound control for each of these two items are different, as discussed in 7-6(f) below. e. Step (5). In this step the Room Factor is determined to obtain the “Rel Spl” as described in Chapter 3. The sound pressure levels in octave bands (Lp) in the room, from one diffuser, is then the total sound power from the diffuser plus the “Rel Spl” as given in equation 3-3. The room volume is 5760 cu. ft., and the acoustic ceiling is 25% of the room surface area. Thin wall surfaces are used on 30% of the room surface area. The term “REL SPL” is determined for a distance of 8 ft. from one diffuser. 7-11 TM 5-805-4/AFJMAN 32-1090 Figure 7-2. Plan View of Supply Duct for Example. Octave Band Center Frequencies 63 125 250 500 1k 2k 4k Room Factor (sq/ft.) 450 600 450 500 600 600 600 “Rel SPL” -9 -11 -9 -10 -11 -11 -11 Total Lw (d. above) 60 58 44 35 40 40 23 Lp-Octave band Sound level 51 47 35 25 29 29 12 f. Step (6). In this step the Lp from e. above is compared with the NC 30 criteria. Octave Band Center Frequencies 63 125 250 500 1k 2k 4k Lp (one diffuser) 51 47 35 25 29 29 12 NC 30 criteria 57 48 41 35 31 29 28 Required Reduction 0 00000 0 This analysis shows that the sound due to the operation of the fan just meets the selected goal in the 125 Hz octave band. In addition the diffuser sound, from one diffuser, would just meet the criteria in the octave band centered at 2000 Hz. However, for this classroom one should also con- sider the total sound from all four diffusers. As the two diffusers closest to the fan are at identical duct distances from the fan the sound of each diffuser can be assumed to be identical. Also the added duct length to the next two diffusers is not 7-12 sufficient to lower the fan noise significantly. Therefore the diffuser noise should be equal for all four outlets. Thus, in the center of the room it is found that the required IL must be increased by the factor of 10log(4), or 6 dB. In this case the sound pressure level in the room for all four diffusers would be: Octave Band Center Frequencies 63 125 250 500 1k 2k 4k Lp (4 diffusers) 57 53 41 31 35 35 18 NC 30 criteria 57 48 41 35 31 29 28 Required Reduction (Considering Four Outlets) 0 5 0 0 4 6 0 To provide the additional IL required for the fan noise in the 125 Hz octave band a 5 ft. long standard pressure drop muffler could be installed in the 60x24 duct in the fan room, or in the 12x24 duct leading to the classroom. The location of choice would depend on the need for sound attenu- ation in other portions of the duct system. The sound in the 1,000 and 2,000 Hz octave bands are due to the diffusers. Mufflers in the duct will not attenuate this sound. For the diffuser noise one solution would be to increase the diffuser size, and this would require changing the diameter of the diffuser drop from a 10 in. to 12 in. diameter yielding lower sound power levels by the order of 8 to 10 dB. TM 5-805-4/AFJMAN 32-1090 CHAPTER 8 VIBRATION CONTROL 8-1. Introduction. This chapter provides the details of vibration isolation mountings so that the desired vibration conditions discussed in chapter 2 can be met for most electrical and mechanical equipment. In addi- tion typical forms of vibration isolators are given, five general types of mounting systems are de- scribed, and summary tables offer suggested appli- cations of five mounting systems for the mechani- cal equipment commonly found in buildings. A discussion of the general consideration for effective vibration isolation is presented in appendix B. 8-2. Vibration Isolation Elements. Table 8-2 lists the principal types of vibration isolators and their general range of applications. This table may be used as a general guide for comparing isolators and their range of static de- flections and natural frequencies as applied to two equipment categories (rotary and reciprocating) and two equipment locations (noncritical and criti- cal). Additional details are required for actual selections of mounts. Vibration isolator types are discussed in this paragraph, and equipment instal- lations are discussed in the remaining paragraphs of this chapter. a. Steel spring isolators. Steel springs are used to support heavy equipment and to provide isola- tion for the typical low-frequency range of about 3 to 60 Hz (180- to 3600-rpm shaft speed). Steel springs have natural frequencies that fall in the range of about 1 Hz (for approximately lo-inch static deflection to about 6 Hz (for approximately 1/4-inch static deflection). Springs transmit high- frequency structureborne noise, so they should be supplemented with a high-frequency pad-type iso- lator when used to support equipment directly over critical locations in a building. Unhoused “stable” steel springs are preferred over housed unstable or stable springs. Unstable springs tend to tilt over when they are loaded and to become short-circuited when they bind against the inside walls of the spring housing. Stable steel springs have a diameter that is about 0.8 to 1.2 times their compressed height. They have a horizontal stiffness that is approximately equal to their verti- cal stiffness; therefore, they do not have a ten- dency to tilt sideways when a vertical load is applied. The free-standing unhoused spring can easily be inspected to determine if the spring is compressed correctly, is not overloaded to the point that adjacent coils are solid against one another, and is not binding against its mounting bracket, and to ensure that all springs of a total installa- tion are uniformly compressed and that the equip- ment is not tilting on its base. For reasons of safety, steel springs are always used in compres- sion, not in tension. b. Neoprenein-shear isolators. Neoprene is a long-lasting material which, when properly shaped, can provide good vibration isolation for the conditions shown in table 8-1. Typically, neoprene-in-shear mounts have the appearance of a truncated cone of neoprene bonded to bottom and top metal plates for bolting to the floor and to the supported equipment. The mount usually has an interior hollow space that is conically shaped. The total effect of the shaping is that for almost any direction of applied load, there is a shearing action on the cross section of neoprene. In this shearing configuration, neoprene serves as a vibration isola- tor; hence, the term “neoprene-in-shear.” A solid block of neoprene in compression is not as effective as an isolator. Manufacturers’ catalogs will show the upper limit of load-handling capability of large neoprene-in-shear mounts. Two neoprene-in-shear mounts are sometimes constructed in series in the same supporting bracket to provide additional static deflection. This gives the double deflection mount referred to in table 8-1. c. Compressed glass fiber. Blocks of compressed glass fiber serve as vibration isolators when prop- erly loaded. The manufacturers have several dif- ferent densities available for a range of loading conditions. Typically, a block is about 2-inches thick and has an area of about 10 to 20 in.2. but other dimensions are available. These blocks are frequently used in series with steel springs to remove high-frequency structureborne noise, and they are often used alone, at various spacings, to support floating concrete floor slabs (fig 6-6). The manufacturer’s data should be used to determine the density and area of a block required to achieve the desired static deflection. Unless otherwise indi- cated, a static deflection of about 5 to 10 percent of the uncompressed height is normal. With long- time use, the material might compress an addi- tional 5 to 10 percent of its height. This gradual change in height must be kept in mind during the designing of floating floors to meet floor lines of structural slabs. 8-1 TM 5-805-4/AFJMAN 32-1090 Table 8-1. General Types and Applications of Vibration Isolators. 8-2 TM 5-805-4/AFJMAN 32-1090 d. Ribbed neoprene pads. Neoprene pads with ribbed or waffle-pattern surfaces are effective as high frequency isolators in series with steel springs. In stacks of 2 to 4 thicknesses, they are also used for vibration isolation of flow power rotary equipment. The pads are usually about 1/4 to 3/8 inches thick, and they compress by about 20 percent of their height when loaded at about 30 to 50 lb/in2. Higher durometer pads may be loaded up to about 100 lb/in2. The pads are effective as isolators because the ribs provide some shearing action, and the spaces between the ribs allow lateral expansion as an axial load is applied. The manufacturer’s literature should be used for proper selection of the material (load-deflection curves, durometer, surface area, height, etc.). e. Felt pads. Felt strips or pads are effective for reducing structureborne sound transmission in the mounting of piping and vibrating conduit. One or more layers of 1/8 or 1/4 inch thick strips should be wrapped around the pipe under the pipe clamps that attach the piping to building structures. Felt pads will compress under long time and high load application and should not be used alone to vibra- tion isolate heavy equipment. f. Cork pads. Cork pads, strips, or blocks may be used to isolate high frequency structureborne noise, but they are not recommended for high load bearing applications because cork gradually com- presses under load and loses its resilience. High density construction cork is sometimes used to support one wall of a double wall. In this applica- tion, the cork will compress slightly with time, and it will continue to serve as a high frequency isolator (say, for structureborne noise above about 100 to 200 Hz), but it will not provide good low frequency isolation at equipment driving fre- quencies of about 10 to 60 Hz. Years ago, before other resilient materials came into widespread use, cork was often misused under heavy vibrating equipment mounts: full area cork pads were fre- quently loaded at rates of 1 to 5 lb/in2. This is such a low loading rate that the cork appears stiff and does not provide the desired resilience. If cork is to be used for vibration isolation, a load deflec- tion curve should be obtained from the supplier, and the cork should be used in the central linear region of the curve (possibly loaded at about 10 to 20 lb/in2). With this loading, the compressed mate- rial will have an initial deflection of about 5% and will continue to compress gradually with age. g. Air springs. Air springs are the only practical vibration isolators for very low frequencies, down to about 1 Hz or even lower for special problems. An air mount consists of pressurized air enclosed in a resilient reinforced neoprene chamber. The air is pumped up to the necessary pressure to carry its load. Since the chamber is subject to very slow leakage, a system of air mounts usually includes a pressure sensing monitor and an air supply (either a pump or a pressurized air tank). A group of air mounts can be arranged to maintain very precise leveling of a base by automatic adjustment of the pressure in the various mounts. If air mounts are used in a design, an active air supply is required. Operational data should be obtained from the manufacturer, 8-3. Mounting Assembly Types. In this paragraph, five basic mounting systems are described for the vibration isolation of equipment. These mounting systems are applied to specific types of equipment in paragraph 8-6. Certain general conditions relating to all the systems are first mentioned. a. General conditions. (1) Building uses. Isolation recommendations are given for three general equipment locations: on grade slabs, on upper floors above noncritical areas, and on upper floors above critical areas. It is assumed that the building under consideration is an occupied building involving many spaces that would require or deserve the low noise and vibra- tion environments of such buildings as hotels, hospitals, office buildings, and the like, as charac- terized by categories 1 through 4 of table 2-1. Hence, the recommendations are aimed at provid- ing low vibration levels throughout the building. If a building is intended to serve entirely such uses as those of categories 5 and 6 of table 2-1, the recommendations given here are too severe and can be simplified at the user’s discretion. An on-grade slab usually represents a more rigid base than is provided by a framed upper floor, so the vibration isolation recommendations can be re- laxed for on-grade installations. Of course, vibra- tion isolation treatments must be the very best when a high-quality occupied area is located im- mediately under the MER, as compared with the case where a “buffer zone” or noncritical area is located between the MER and the critical area. (2) Structural ties, rigid connections. Each piece of isolated equipment must be free of any structural ties or rigid connections that can short- circuit the isolation joint. (a) Electrical conduit should be long and “floppy” so that it does not offer any resistance or constraint to the free movement of the equipment. Piping should be resiliently supported. Limit stops, shipping bolts, and leveling bolts on spring isola- tors should be set and inspected to ensure that 8-3 TM 5-805-4/AFJMAN 32-1090 they are not inadvertently short-circuiting the spring mounts. (b) All building trash should be removed from under the isolated base of the equipment. Loose pieces of grout, 2x4s, nuts, bolts, soft drink bottles, beer cans, welding rods, pipes, and pipe couplings left under an equipment base can short- circuit the isolation mounts. It is recommended that a 2 inch to 4 inch clearances be provided under all isolated equipment in order to facilitate inspection and removal of trash from under the base. (c) For many equipment installations, there is no need to bolt down the isolation mounts to the floor because the smooth operation of the machine and the weight of the complete assembly keep the system from moving. For some systems, however, it may be necessary to restrain the equipment from “creeping” across the floor. In these situa- tions, it is imperative that the hold-down bolts not short circuit the pads. A suggested restraining arrangement is illustrated in figure 8-1. Simpler versions can be devised. (d) For buildings located in earthquake- prone areas, the isolation mounts should contain snubbers or motion-limiting devices that restrain the equipment against unusual amounts of move- ment. These snubbers should be set to provide adequate free movement for normal equipment operation. These devices are available from most suppliers of isolator equipment. b. Type I mounting assembly. The specified equipment should be mounted rigidly on a large integral concrete inertia block. (Unless specified otherwise, all concrete referred to in this manual should have a density of at least 140 to 150 lb/ft. 3 .) (1) The length and the width of the inertia block should be at least 30 percent greater than the length and width of the supported equipment. (2) Mounting brackets for stable steel springs should be located off the sides of the inertia block at or near the height of the vertical center-of- gravity of the combined completely assembled equipment and concrete block. If necessary, curbs or pedestals should be used under the base of the steel springs in order to bring the top of the loaded springs up to the center-of-gravity position. As an alternative, the lower portion of the concrete iner- tia block can be lowered into a pit or cavity in the floor so that the steel springs will not have to be mounted on curbs or pedestals. In any event, the clearance between the floor (or all the surfaces of the pit) and the concrete inertia block shall be at least 4 inches, and provision should be allowed to check this clearance at all points under the block. (3) Floor slab thickness. It is assumed that MER upper floor slabs will be constructed of dense concrete of 140-150 lb/ft. 3 density, or, if lighter concrete is used, the thickness will be increased to provide the equivalent total mass of the specified floor. For large MERs containing arrays of large and heavy equipment, it is assumed that the floor slab thickness will be in the range of 8 to 12 inches, with the greater thicknesses required by the greater floor loads. For smaller MERs contain- ing smaller collections of lighter weight but typi- Figure 8-1. Suggested Arrangement of Ribbed Neoprene Pads for Providing Resilient Lateral Restraint to a Spring Mount. 8-4 TM 5-805-4/AFJMAN 32-1090 cal equipment, floor slab thicknesses of 6 to 10 inches are assumed. For occasional locations of one or a very few pieces of small high-speed equipment (say 1800 rpm or higher) having no reciprocating action, floor slabs of 4 to 6 inches may be used with reasonable expectation of satisfactory results. However, for reciprocating-action machines operat- ing at the lower speeds (say, under 1200 rpm), any floor slab thicknesses reduced from those listed above begin to invite problems. There is no clear crossover from “acceptable” to “unacceptable” in terms of floor slab thickness, but each reduction in thickness increases the probability of later difficul- ties due to vibration. The thicknesses mentioned here are based on experience with the “acoustics” of equipment installations. These statements on thicknesses are in no way intended to represent structural specifications for a building. “House- keeping pads” under the equipment are assumed, but the height of these pads is not to be used in calculating the thickness of the floor slab. (4) The ratio of the weight of the concrete block to the total weight of all the supported equipment (including the weight of any attached filled piping up to the point of the first pipe hanger) shall be in accordance with the recommen- dations given in the paragraph and table for the particular equipment requiring this mounting as- sembly. The inertia block adds stability to the system and reduces motion of the system in the vicinity of the driving frequency. For reciprocating machines or for units involving large starting torques, the inertia block provides much-needed stability. (5) The static deflection of the free-standing stable steel springs shall be in accordance with the recommendations given in the paragraph and ta- ble for the particular equipment. There shall be adequate clearance all around the springs to as- sure no contact between any spring and any part of the mounted assembly for any possible align- ment or position of the installed inertia block. c. Type II mounting assembly. This mount is the same as the Type I mount in all respects except that the mounting brackets and the top of the steel springs shall be located as high as practical on the concrete inertia block but not necessarily as high as the vertical center-of-gravity position of the assembly, and the clearance between the floor and the concrete block shall be at least 2 inches. (1) If necessary, the steel springs can be re- cessed into pockets in the concrete block, but clearances around the springs should be large enough to assure no contact between any spring and any part of the mounted assembly for any possible alignment or position of the installed inertia block. Provision must be made to allow positive visual inspection of the spring clearance in its recessed mounting. (2) When this type of mounting is used for a pump, the concrete inertia block can be given a T-shape plan, and the pipes to and from the pump can be supported rigidly with the pump onto the wings of the T. In this way, the pipe elbows will not be placed under undue stress. (3) The weight of the inertia block and the static deflection of the mounts shall be in accord- ance with the recommendations given in the table for the particular equipment. d. Type III mounting assembly. The equipment or the assembly of equipment should be mounted on a steel frame that is stiff enough to allow the entire assembly to be supported on flexible point supports without fear of distortion of the frame or misalignment of the equipment. The frame should then be mounted on resilient mounts-steel springs or neoprene-in-shear mounts or isolation pads, as the static deflection would require. If the equip- ment frame itself already has adequate stiffness, no additional framing is required, and the isola- tion mounts may be applied directly to the base of the equipment. (1) The vibration-isolation assembly should have enough clearance under and all around the equipment to prohibit contact with any structural part of the building during operation. (2) If the equipment has large starting and stopping torques and the isolation mounts have large static deflections, consideration should be given to providing limit stops on the mounts. Limit stops might also be desired for large deflec- tion isolators if the filled and unfilled weights of the equipment are very different. e. Type IV mounting assembly. The equipment should be mounted on an array of “pad mounts”. The pads may be of compressed glass fiber or of multiple layers of ribbed neoprene or waffle- pattern neoprene of sufficient height and of proper stiffness to support the load while meeting the static deflection recommended in the applicable accompanying tables. Cork, cork-neoprene, or felt pad materials may be used if their stiffness char- acteristics are known and if they can be replaced periodically whenever they have become so com- pacted that they no longer provide adequate isola- tion. (1) The floor should be grouted or shimmed to assure a level base for the equipment and there- fore a predictable uniform loading on the isolation pads. (2) The pads should be loaded in accordance with the loading rates recommended by the pad 8-5 TM 5-805-4/AFJMAN 32-1090 manufacturer for the particular densities or duro- meters involved. In general, most of these pads are intended for load rates of 30 to 60 psi, and if they are underloaded (for example, at less than about 10 psi), they will not. be performing at their maximum effectiveness. f. Type V mounting assembly (for propeller-type cooling towers). Large, low-speed propeller-type cooling towers located on roof decks of large buildings may produce serious vibration in their buildings if adequate vibration isolation is not provided. In extreme cases, the vibration may be evident two or three floors below the cooling towers. (1) It is recommended that the motor, drive shaft, gear reducer, and propeller be mounted as rigidly as possible on a “unitized” structural sup- port and that this entire assembly be isolated from the remainder of the tower with stable steel springs in accordance with table 8-8. Adequate clearance between the propeller tips and the cool- ing tower shroud should be provided to allow for starting and stopping vibrations of the propeller assembly. Several of the cooling tower manufactur- ers provide isolated assemblies as described here. This type of mounting arrangement is shown schematically in figure 8-2. (2) In addition, where the cooling tower is located on a roof deck directly over an acoustically critical area, the structureborne waterfall noise may be objectionable; it can be reduced by locating three layers of ribbed or waffle-pattern neoprene between the base of the cooling tower and the supporting structure of the building. This treat- ment is usually not necessary if there is a noncri- tical area immediately under the cooling tower. (3) A single-treatment alternate to the com- bined two treatments of (1) and (2) above is the isolation of the entire cooling tower assembly on stable steel springs, also in accordance with table 8-8. The springs should be in series with at least two layers of ribbed or waffle-pattern neoprene if there is an acoustically critical area immediately below the cooling tower (or within about 25 feet horizontally on the floor immediately under the tower). It is necessary to provide limit stops on these springs to limit movement of the tower when it is emptied and to provide limited movement under wind load. (4) Pad materials, when used, should not be short-circuited by bolts or rigid connections. A schematic of an acceptable clamping arrangement for pad mounts is shown in figure 8-3. Cooling tower piping should be vibration-isolated in accord- ance with suggestions given for piping. 8-4. Tables Of Recommended Vibration Isola- tion Details. a. Table format. A common format is used for all the tables that summarize the recommended vibration isolation details for the various types of equipment. A brief description of the format is given here. (1) Equipment conditions. The three columns on the left of the table define the equipment conditions covered by the recommendations: loca- tion, rating, and speed of the equipment. The rating is given by a power range for some equip- ment, cooling capacity for some, and heating ca- pacity for some. The rating and speed ranges generally cover the range of equipment that might be encountered in a typical building. Subdivisions in rating and speed are made to accommodate variations in the isolation. If vibrating equipment is supported or hung from an overhead floor slab, immediately beneath an acoustically critical area, the same degree of vibration isolation should be provided as is recommended for the location desig- nated as “on upper floor above critical area”. Similarly, if the vibrating equipment is hung from an overhead floor slab beneath a noncritical area, the same vibration isolation should be provided as is recommended for the location designated as “on upper floor above noncritical area”. Figure 8-2. Schematic of Vibration Isolation Mounting for Fan and Drive-Assembly of Propeller-Type Cooling Tower. 8-6 [...]... each piece of equipment b Centrifugal and axial-flow fans The recommended vibration isolation mounting for fans are given in table 8-2 Ducts should contain flexible connections at both the inlet and discharge of the fans, and all connections to the fan assembly should be clearly flexible The entire assembly should bounce with little restraint when one jumps up and down on the unit Where supply fan... Reciprocating-compressor refrigeration equip ment The recommended vibration isolation for this equipment are given in table 8-3 These recommendations apply also to the drive unit used with the reciprocating compressor Pipe connec8 -7 TM 5-805-4/AFJMAN 32-1090 Table 8-2 Vibration Isolation Mounting for Centrifugal and Axial-Flow Fans 8-8 TM 5-805-4/AFJMAN 32-1090 Table 8-3 Vibration Isolation Mounting for Reciprocating... and down on the unit Where supply fan assemblies are located over critical areas, it is desirable to install the entire inlet casing and all auxiliary equipment (coil decks and filter sections) on floated concrete slabs The floated slab may also serve to reduce airborne noise from the fan inlet area into the floor area below Large ducts (cross-section area over 15 sq feet) that are located within about... static deflection of the isolators is being considered, these minimum values are keyed to the approximate span of the floor beams; that is, as the floor span increases, the floor deflection increases, and therefore the isolator deflection must increase The specific minimum deflection in effect specifies the type of isolator that can be used; refer to table 8-1 for the normal range of static deflection... recommendations: Column 1, the type of mounting; Column 2, the suggested minimum ratio of the weight of the inertia block (when required) to the total weight of all the equipment mounted on the inertia block; and Column 3, the suggested minimum static deflection of the isolator to be used (a) When the weight of the inertia block is being considered, the larger weight of the range given should be applied where . 13 Drop 7 5 9 16 32 41 43 24 16 10 6 15 22 37 52 53 35 23 Standard 3 3 6 13 23 33 33 23 14 Pressure 5 4 11 20 37 44 44 36 22 Drop 7 6 13 28 42 47 47 45 30 10 8 15 35 50 56 57 57 40 Octave Band Center. bands to pro- vide the total duct attenuation in dB in each octave band. Octave Band Center Frequencies 63 125 250 500 lk 2k 4k 20' of 24x60 3 4 7 18 32 30 30 9’ of 12x24 3 2 5 12 31 27. isolators and their range of static de- flections and natural frequencies as applied to two equipment categories (rotary and reciprocating) and two equipment locations (noncritical and criti- cal).